FIG. 10 shows a stator structure that is incorporated in a conventional motor, and in this Figure, reference numeral 1 represents a stator main body, 2 represents a rotor, 4 represents a slot and 17 represents a tooth having a widened portion on its tip. Here, this Figure is a partial drawing that is ¼ divided, and actually, both the stator main body and the rotor have a 360-degree circumferential shape. Moreover, this Figure illustrates a winding specification per one phase.
In the stator structure shown in FIG. 10, the stator main body 1 has a plurality of slots 4 and teeth 17, and a coil (not shown) is wound on each of the teeth 17 in a concentrated winding system to finally form a stator. Further, a rotor 2 having a gear shape is maintained on the circumference of the stator main body 1 so as to freely rotate concentrically with the stator 1, in a manner so as to face the stator main body 1 through an air gap 19 formed on the inner circumferential side thereof.
The motor having a stator structure of this type is referred to as a reluctance motor, and, for example, the reluctance motor shown in FIG. 10, which has eighteen slots in the stator main body with twelve teeth of the rotor, is referred to as a 18/12 reluctance motor.
The reluctance motor is constituted by only laminated electromagnetic steel plates and coils, and the number of the teeth is eighteen that is the same as the number of the slots, and each of the teeth is individually provided with a coil in the concentrated winding system.
When a coil current is allowed to flow in a direction shown in FIG. 10 (symbol ∘ indicates a direction of current flowing from the rear side to the surface side of the Figure, and symbol ∘X indicates a direction of current flowing from the surface side to the rear side of the Figure), the rotor 2 is attracted by a magnetomotive force generated by the coil current of the stator main body 1 and, in the example of FIG. 10, a torque is generated clockwise so that it is allowed to continuously rotate by switching the phase of the current to be applied in accordance with the rotation position of the rotor. Formula (1) shows a torque formula of the reluctance torque.
                    T        =                              1            2                    ⁢                      i            2                    ⁢                                    ∂              L                                      ∂              θ                                                          (        1        )            
In formula (1), “T” represents a torque, “i” represents a current, “L” represents a winding inductance and “θ” represents a rotation position. As clearly shown by formula (1), torque “T” is proportional to the square of current “i”, and is also proportional to the rate of change of winding inductance “L” by rotor position “θ”.
In the stator main body 1 of a conventional reluctance motor, with respect to the width of the tooth 17, a tooth width “W2” on the side facing the air gap 19 of the rotor 2 outside the main body is wider than a tooth width “W1” on the slot bottom face side.
The reason that the tooth width “W2” on the surface side facing the air gap 19 is made wider is because this structure prevents the coil wound inside the slot from jumping out of the slot 4 during the rotation of the motor, and also makes it possible to increase the amount of magnetic flux that enters the stator main body 1 from the rotor 2 through the air gap 19.
FIG. 11 shows a current-torque characteristic of the conventional reluctance motor having the stator structure shown in FIG. 10. In this FIG. 11, the theoretical value, calculated from formula (1), is indicated by a broken line, and the measured value is indicated by a solid line. Since the torque is proportional to the square of the current as shown in formula (1), the torque characteristic is represented by the theoretical value; however, actually, the torque increasing rate becomes lower as the current increases, resulting in a characteristic as indicated by the solid line.
However, in the above-mentioned conventional reluctance motor, as the current increases, the torque increasing rate becomes lower, with the result that the torque decreases in comparison with the theoretical value.
This is because when the current increases due to an excessive load, a magnetic saturation occurs in the rotor and stator to cause a reduction in inductance “L”.
In recent years, along with growing consciousness for the global environment, various equipments that take this point into consideration have been proposed, and many products of this type have been supplied to the market. In the field of automobiles also, automobiles, which use motors and an internal combustion engine using conventional fossil fuel or use only motors, that is, so-called electric automobiles, have been developed and sold.
The major subject with such automobiles is how to improve the mileage, and with respect to the motor to be used therein, there have been strong demands for small size and light weight.
Therefore, with respect to motors to be used with short-time rating, for example, those used in ABS devices and the like, in the case of the motor that causes a torque reduction when an attempt is made to obtain a predetermined torque, it is necessary to increase the thickness of laminated layers or the like, and this causes an increase in the weight, and the subsequent increase in the weight of the automobile or the like to carry the motor. Consequently, the mileage is lowered.
Moreover, the motor has an abrupt temperature rise due to use with short-time rating, resulting in degradation in reliability of insulating materials and the like.